Dual linear/circular polarization spiral antenna



March 12, 1968 L. L. BLAISDELL 3,373,433 7 DUAL LINEAR/CIRCULAR POLARIZATION SPIRAL ANTENNA Filed Dec. 16; 1964 Fla. 2 SPIRAL WINDINGS A B 15k lb Fla. 3 SPIRAL 2O 21 WINDINGS A a 22 22 14 j o SIGNAL f 24 SOURCE 26 i [14 SIGNAL SOURCE INVENTOF LEONARD L. BLAISDELL ATTORNEY United States Patent 3,373,433 DUAL LINEAR/ CIRCULAR POLARIZATION SPIRAL ANTENNA Leonard L. Blaisdell, Medway, Mass., assignor to Sylvania Electric Products Inc., a corporation of Delaware Filed Dec. 16, 1964, Ser. No. 418,839 2 Claims. (Cl. 343850) ABSTRACT OF THE DISCLOSURE This invention relates generally to antenna systems, and more particularly to a spiral antenna system wherein dual linear/circular polarization is provided by a singlespiral antenna.

The spiralantenna is wellv known in the art, and is widely used to transmit circularly polarized waves. In

one well known version, the antenna comprises a pair of bifilar spirally wound conductors of equal length.

When the inner ends of the conductors are energized with anti-phase signals, circularly polarized radiation results having the same sense as that of the spirally wound conductors. Radiation of opposite sense results when the outer ends of the conductor are energized. Another version of thespiral antenna employs a pair of bifilar spirally wound conductors with the spiral windings diifering in lengthlby one-quarter wavelength. This antenna produces circularly polarized radiation when energized with in-phasesignals applied to the conductors.

It would be advantageous in many instances to be able to generate linearly polarized, as well as circularly polar- :ized, radiation from the same antenna. One method for providing such polarization diversity employs a spiral an- .tenna which is simultaneously energized at its inner and outer ends to thereby produce linearly polarized radiajtion. This antenna, however, requires extremely careful phasing and balancing of the input signals in order to produce linear polarization and, therefore, is a relatively complex and expensive means of accomplishing the desired result. Another known system employs two spiral antennas each wound with opposite sense, and simulta-' "neously energized to produce a linearly polarized far field radiation pattern. Still another known system produces linear polarization by means of a spiral antenna operated in conjunction with a reflector and parasitic elements. These last-mentioned systems are rather complex and their practical applications limited by reason of their complexity, cost and size. In addition, these last-mentioned systems, while capable of producing linear polarization,

are not readily adaptable to providing circular polariza- 3,373,433 Patented Mar. 12, 1968 tion. It is, therefore, a primary object of the present invention to provide a simple antenna system capable of producing both linear and circular polarization.

Another object of the invention is to provide a spiral antenna capable of producing linear and circular polarization with only a single energizing source.

A further object of the invention is to provide an antenna system wherein the sense of linear polarization is easily varied.

Applicant has discovered that a spiral antenna can provide linear polarization when energized with a quadrature phased signal having both in-phase and out-of-phase components of equal amplitude. The in-phase signal components produce circular polarization of one sense, while the out-of-phase signal components produce circular polarization of opposite sense, the two senses of circular polarization combining to produce the desired linear p0- larization. Applicant has also discovered that by altering the relative polarity of the applied energizing signals, the sense of the linear polarization can be correspondingly varied by By the use of a simple switching device, the antenna can also produce either sense of circular polarization.

The invention will be more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic representation of a spiral antenna of the type useful in the present invention;

FIG. 2 is a schematic representation of an antenna system accordingto the invention; and

FIG. 3 is a schematic representation of another embodiment of the invention.

Reiierring to FIG. 1, there is shown a spiral antenna having inner feed points A and B, equally spaced about the center 0 of the spiral configuration. The inner feed points -A and B are respectively connected to two spiral Windings AA and BB, where the length of the spiral Winding AA is a quarter of a wavelength longer than the spiral winding BB. When balanced anti-phase (0-180) signals are applied to inner feed points A and B, respectively, no radiation occurs at the feed points because the signals are out of phase and there is no circulating current. As the signal currents in the respective spiral windings progress outward from the feed points, the currents gradually become in-phase in adjacent windings, until at a radius equal to 1(21r) the currents are precisely in phase and maximum radiation occurs. This produces one sense of circular polarization as seen in the far field. Conversely, if balanced in-phase (0-0) signals are applied to feed points A and B, respectively, no radiation occurs at the feed points because of the lack of circulating currents. As the signals in the respective windings progress outward along the spiral, no radiation occurs during the outward progression since the currents in adjacent windings are never in-phase, and the signals reach the ends of the windings practically undiminshed in amplitude. The signals are reflected backward along the v spiral windings and, because of the quarter wavelength drflerence in the length of the windings, the signal on winding AA travels a half wavelength further than the signal on winding BB. Therefore, during the inward progression of the currents along the spiral windings, the current in the adjacent windings gradually become inphase, until at a radius equal to 1-(21r)- the currents are precisely in-phase and maximum radiation occurs. Because the radiation occurs during the inward progression of the signals on the spiral windings with --O feed, the sense of circular polarization produced in opposite to the sense produced with 0180 feed.

Applicant has now discovered that linear polarization can be produced by employing an energizing signal having both in-phase and anti-phase components, such as a quadrature phased signal. The in-phase components produce One sense of circular polarization, while the anti phase components produce the opposite sense of circular polarization, and both senses of circular polarization then combine to produce the desired linear polarization.

An antenna system embodying the invention is illus trated in FIG. 2. The spiral windings are shown (in side view) mounted on a suitable backing material 10, for example by etched circuit techniques, with input lines 11 and 12 connected to inner feed points A and B, respectively. A quadrature feed device, such as quadrature hybrid 13, is connected between input terminals 20 and 21 and output terminals 30 and 31, and includes a pair of transmission lines 15 and 16 interconnected at quaterwave intervals by lines 17, 18 and 19. The output terminals 30 and 31 of quadrature hybrid 13 are connected to respective feed lines 11 and 12.

In operation, a signal from source 14 is applied via line 24 to a two position switch 22(22'). In switch position 22, the output of signal source 14 is applied to input terminal 20 of the quadrature hybrid; in switch position 22, the output of' the signal source is applied to input terminal 21 of the quadrature hybrid.

When a signal from signal source 14 is applied to input terminal 20, a pair of quadrature phased signals appear at hybrid terminals 30 and 31, by action of quadrature hybrid 13, and are applied, respectively, to antenna terminals A and B via lines 11 and 12. A signal applied to input terminal 22 also produces a pair of quadrature signals at terminals 30 and 31; however, the sense of the signals is opposite to that produced via input terminal 22, since the polarity of the quadrature signals is reversed.

Since in either of the above cases the signals applied to the inner feed points A and B are out-of-phase by 90, it will be appreciated that such signals are devisable into two pairs of component signals, wherein one pair of component signals are in-phase (0--0) and one pair of component signals are anti-phase (0-180). As described previously, the components signals which are antiphase (0-l80) gradually become in-phase during the outward progression along the spiral windings and are precisely in-phase on adjacent windings at a radius equal to 1'(21r)' where radiation having one sense of circular polarization occurs. The signal components which are initially in-phase (00) progress outward on the spiral windings and are reflected back at the ends of the windings and become in-phase again at a radius equal to 1-(21r)- but, because they are traveling in the opposite direction, produce circular polarization of opposite sense to that produced by the anti-phase signal components. The two opposite senses of circular polarization combine in the far field to produce linear polarization. The attitude of the linear polarization can be easily varied by switch 22. When the switch is connected to input terminal 20, the liner polarization produced by the antenna is orthogonal to that produced when the switch is connected to input terminal 21 of quadrature hybrid 13, due to the opposite polarity of the energizing signals.

Another embodiment of the invention is illustrated in FIG. 3 by which circular polarization can be provided, in adidtion to linear polarization, and includes a 90 phase shifting device 25, for example a quarter-wave line, connected between output terminal 31 of quadrature hybrid 13, and input line 12 of the spiral antenna, and having a 4 switch 26 connected across the input and output terminals. The switch may be, for example, a simple diode switch. When switch 26 is open, phase shifter 25 is in the transmission path from terminal 31 to inner feed point B, while the phase shifter is shorted out of this transmission path when switch 26 is closed. Phase shifting device 25 is designed to be relatively frequency independent over the bandwidth of operation of the antenna system.

When switch 26 is closed, the operation of the system of FIG. 3 is the same as that of FIG. 2. When switch 26 is open, however, and a signal is applied to input terminal 20 of the quadrature hybrid, one portion of the signal is applied directly to inner feed point A of the spiral antenna element, while the other portion of the signal is delayed by by action of quadrature hybrid 13, and is delayed by an additional 90 by. phase shifting device 25 before reaching inner feed point B. As a result, the signal at point B isv out-of-phasewith the signal at point A, causing the antenna to radiate circular polarization. Conversely, when the signal is applied to input terminal 21 (switch position 22'), they signal applied to point A is delayed by 90 by quadrature hybrid 13, and the signal applied to point B is delayed by 90 by phase shifter 25, so that the signals at points A and B, are in-phase' (0-0), causing circular polarization of opposite sense. Thus, utilizing the configuration of FIG. 3 with switch 26 open, it is possible to produce either sense of circular polarization, while with switch 26 in the closed position, linear polarization having one of two senses is produced.

From the foregoing, it is. readily apparent that a spiral antenna system has been provided which can produce dual linear/circular polarization. Utilizing simple switching techniques, this system can produce either sense of circular polarization, as well as either of two orthogonal linear polarizations. The utility of the invention is enhanced by its relatively simple structure, since it requires only a single spiral antenna element, a single signal source, and relatively simple feeding devices for transmitting the signal from its source to the antenna element.

While particular embodiments of the present invention have been shown and described, many modifications and variations will become obvious to those skilled in the are without departing from the true spirit of the, invention. For example, many ditferent forms of feeding devices to produce the desired quadrature phase signals, are available. In addition, switch 26 and phase shifting device 25 of FIG. 3 may take a variety of forms to meet particular design requirements. The invention, therefore, is to be limited only to the extent of the. following claims.

What is claimed is:

1. An antenna system comprising, in combination, a spiral antenna having first and second spiral-windings differing in length by approximately one quarter. Wavelength at the operating frequency and each having an inner feed point, a phase shifting device. having an input terminal and an output terminal, means connecting the output terminal of said phase shifting device to the inner feed point of said second spiral winding, a quadrature feed device having first and second input terminals and first and second output terminals, means connecting the first output terminal of said quadrature feed device to the inner feed point of said first spiral winding, means connecting the second output terminal of said quadrature feed device to the input terminal of said phase shifting device, means for applying a direct connection between the input terminal and the output terminal of said phase shifting device, and means for selectively applying a signal to the first or the second input terminal of said quadrature feed device.

2. A spiral antenna for selectively providing linearly or circularly polarized radiation, comprising:

first and second bifilar spirally wound conductive windings differing in length by one quarter wavelength at the operating frequency and each having an inner feed point, said first winding being longer than said second Winding;

a quadrature feed device having a first output coupled to the feed point of said first Winding and a second output coupled to the feed point of said second winding;

a 90 phase shifter;

means for selectively connecting said phase shifter between the second output of said quadrature feed device and the feed point of said second winding; and

means for applying an energizing signal to an input terminal of said quadrature feed device.

References Cited UNITED STATES PATENTS Gothe 343853 Donnellan et al. 343895 X Wheeler 343908 X Fink 343-854 X Kaiser et al. 343854 Butler 343854 10 ELI LIEBERMAN, Primary Examiner. 

